In typical batch type industrial heat treat furnaces operated at positive pressures, workpieces are loaded into a wire mesh work tray which is moved into the furnace chamber for the heat treat process and thence out of the furnace chamber into a quench chamber or a vestibule. When the tray is loaded, it is quite heavy, often in excess of 1,000 lbs and movement of the loaded tray into and out of the furnace chamber can become difficult. It is thus conventional to imbed rollers into the refractory floor or hearth in the furnace chamber and use a drive chain arrangement to automatically pull or push the loaded work tray into and out of the furnace chamber. Such arrangements are conventionally referred to as roller rail hearths.
Batch type heat treating furnaces operated at positive pressures are general, all purpose type furnaces which are designed so that a wide configuration of different parts can be heat treated in accordance with any number of different heat treat processes. Such furnaces fundamentally require an integral box type furnace chamber which is soundly insulated in contrast to other types of furnaces which may have certain structural provisions for treating particular part configurations or applicability for only certain type of heat treat processes.
No matter what heat treat process is employed but particularly for high temperature processes such as carburizing, it is important from an economic (as well as possibly from a process) point of view to heat the work to its heat treat temperature as quickly as possible. In this direction, efforts have been made to produce higher and higher furnace heating rates with, for example, gas burners so as to reduce the time it takes to raise the temperature of the work to its heat treat temperature. At present, it is not uncommon, for example in a batch type carburizing process, to supply heat to the furnace chamber from the radiant tubes which are at temperatures of between 1950.degree. and 2050.degree. F. When the conventional roller hearth is subjected to such temperatures, the thermal stress induced in the roller support arrangement coupled with the heavy work loads easily cause permanent distortion of the roller support. To avoid such distortion, the furnace manufacturers have been forced to reduce the capacity of the furnace as a way to insure that the overall stress level on the roller supports does not exceed the elastic limit of the material.
Movable hearths are known in the furnace art. Conventional rotary hearths use doughnut shaped refractory beds which rotate within a fixed housing as the work deposited thereon is sequentially heated in a predetermined manner as it passes through several fixed stations within the hearth. Another movable hearth arrangement is car bottom furnaces which are conventionally used in steel mill applications, for annealing and tempering. In such arrangements, a cart rolling on a rail is actually rolled under a bottomless furnace enclosure which is then sealed to the cart to form the furnace enclosure. Such arrangements typically use sand seals to establish the furnace enclosure between the car bottom and the furnace walls which, while perfectly acceptable in steel mill applications are not adequate for the repeated loadings encountered in batch type industrial heat treat furnace arrangements.
Nevertheless, within the literature, variations on the car bottom approach, using sand seals, can be found in the art such as in U.S. Pat. No. 1,946,270 to Breaker and U.S. Pat. No. 2,869,856 to Greene both of which use a hydraulic ram to lift a hearth into the open bottom of a furnace chamber in an industrial furnace application. In addition, once the hearth is lowered, all the furnace temperatures are released rendering such devices uneconomical. A further variation on the car bottom furnace may be found in U.S. Pat. No. 4,421,481 to Holz et al in which cars are rolled into an enclosure end to end for the stated purpose of forming a hearth. Holz is relevant to the present invention only in the sense that some form of a movable hearth is thus disclosed.